The present invention relates to two-photon absorbing, benzobisthiazole-based chromophores with high oleophilicity as indicated by their high solubility in hydrocarbon solvents.
Multiphoton absorption (MPA) is a nonlinear optical process that occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium, with the former being more common. For a given chromophore, these absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of two-photon absorption (2PA), two quanta of photons may be absorbed from a single light source (degenerate 2PA) or two sources of different wavelengths (non-degenerate 2PA). In considering the practical exploitation of 2PA process, it is important to recognize the following useful features of the 2PA phenomenon, based on the fact that 2PA scales nonlinearly with the squared intensity of the incident laser beam: (i) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, near infrared (NIR) to ultraviolet (UV) upconversion; (ii) deeper penetration of incident NIR light than UV light that may be hazardous with prolonged exposure; (iii) highly localized excitation as compared with one-photon processes allowing for precise spatial control of in situ photochemical or photophysical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching; and (iv) fluorescence (i.e. light emission via relaxation from singlet excited state to ground state) when properly manipulated, that would allow for information/signal feedback or amplification in conjunction with other possible, built-in effects such as surface plasmonic enhancement effect. It is anticipated that further ingenious utilization of these basic characteristics will lead to practical applications other than those already emerged in such diverse areas as bio-medical fluorescence imaging, data storage, directed energy protection, hazardous chemical detection, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc. In the past two decades, significant advances have been made in the fundamental understanding of structure-property relationship that has led to the design and synthesis of two-photon absorbers with very large cross-section values. Although further enhancement of 2PA cross-section is still possible as suggested by a number of theoretical studies, for certain applications, the two-photon-property requirement has essentially been met by the state-of-art chromophores. Because of the possible structure-property-processing trade-off, which requires certain balancing, the secondary properties, i.e. thermal and mechanical properties as well as practical solubility in aqueous and/or common organic solvents, are especially important to material processing into various useful forms (films, coatings, fibers, windows etc.) and device configurations.
In the inventors' previous work as documented in U.S. Pat. No. 6,730,793, Ramamurthi Kannan et al, issued May 4, 2004, a suite of quadrupolar and octupolar two-photon absorbing compounds based on the “Donor-Acceptor-Donor” structural motif were claimed. Among these 2PA compounds were quadrupolar examples based on diphenylamine-benzobisthiazole-diphenylamine motif, which have the attractive properties of high two-photon absorptivity and easily accessible starting material for the construction of the benzobisthiazole structure as the electron-accepting core.
Accordingly, it is an object of the present invention to provide a new series of highly hydrocarbon-soluble, two-photon absorbing compounds with a common 2,6-benzobisthiazolyl core with both sides connected to 9,9-dialkylfluorenyl moieties, and endcapped with extended arylamino groups. They have been synthesized via a Suzuki cross-coupling reaction of a new benzobisthiazole-containing dibromide precursor and a triphenylamine(boronate ester) and amination of the same bromo precursor with diphenylamine. These conjugated aromatic-heterocyclic compounds show high solubility in nonpolar hydrocarbon solvents (oleophilicity) and high two-photon properties, especially in the nanosecond domain of pulse-laser excitation.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.